Add openid-configuration URL for configuring OpenID Connect clients

What does this MR do?

This MR cherry-picks a contribution by @nils.werner to a new MR. We had trouble getting the rebase to work in the original MR (and the pipelines were failing), so this MR replaces the original.

Original MR: !45029 (closed)

Original description: This MR adds a small paragraph mentioning the /.well-known/openid-configuration URL to automate configuring OpenID Connect clients.

• This change was approved by @rymai in the original: !45029 (comment 749501000)
• The Technical Writing review is complete.
• I shortened the MR title and commit message from the original, to ensure that the Danger review passes.

All that remains is to merge this contribution.

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